Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 28 Aug 2020 13:12:21 +0200
From:   Marco Elver <elver@google.com>
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        kasan-dev@googlegroups.com,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Elena Petrova <lenaptr@google.com>,
        Branislav Rankov <Branislav.Rankov@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 35/35] kasan: add documentation for hardware tag-based
 mode
Message-ID: <20200828111221.GA185387@elver.google.com>
References: <cover.1597425745.git.andreyknvl@google.com>
 <5d0f3c0ee55c58ffa9f58bdea6fa6bf4f6f973a4.1597425745.git.andreyknvl@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5d0f3c0ee55c58ffa9f58bdea6fa6bf4f6f973a4.1597425745.git.andreyknvl@google.com>
User-Agent: Mutt/1.14.4 (2020-06-18)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Aug 14, 2020 at 07:27PM +0200, Andrey Konovalov wrote:
> Add documentation for hardware tag-based KASAN mode and also add some
> clarifications for software tag-based mode.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  Documentation/dev-tools/kasan.rst | 73 +++++++++++++++++++++----------
>  1 file changed, 51 insertions(+), 22 deletions(-)
> 
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index a3030fc6afe5..aeed89d6eaf5 100644
[...]  
> -Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
> -store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
> -uses shadow memory to store memory tags associated with each 16-byte memory
> -cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
> +Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64
> +CPUs to store a pointer tag in the top byte of kernel pointers. Like generic
> +KASAN it uses shadow memory to store memory tags associated with each 16-byte
> +memory cell (therefore it dedicates 1/16th of the kernel memory for shadow
> +memory).

It might be helpful to be more specific vs. saying "modern arm64 CPUs".
Does the "modern" qualifier suggest not all arm64 CPUs support the
feature?  (HW tag-based KASAN below is specific, and mentions ARMv8.5.)

> +On each memory allocation software tag-based KASAN generates a random tag, tags
> +the allocated memory with this tag, and embeds this tag into the returned
> +pointer.
>  
> -On each memory allocation tag-based KASAN generates a random tag, tags the
> -allocated memory with this tag, and embeds this tag into the returned pointer.
>  Software tag-based KASAN uses compile-time instrumentation to insert checks
>  before each memory access. These checks make sure that tag of the memory that
>  is being accessed is equal to tag of the pointer that is used to access this
> -memory. In case of a tag mismatch tag-based KASAN prints a bug report.
> +memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
>  
>  Software tag-based KASAN also has two instrumentation modes (outline, that
>  emits callbacks to check memory accesses; and inline, that performs the shadow
> @@ -215,9 +222,31 @@ simply printed from the function that performs the access check. With inline
>  instrumentation a brk instruction is emitted by the compiler, and a dedicated
>  brk handler is used to print bug reports.
>  
> -A potential expansion of this mode is a hardware tag-based mode, which would
> -use hardware memory tagging support instead of compiler instrumentation and
> -manual shadow memory manipulation.
> +Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses aren't
> +checked).
> +
> +Software tag-based KASAN currently only supports tagging of slab memory.
> +
> +Hardware tag-based KASAN
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Hardware tag-based KASAN is similar to the software mode in concept, but uses
> +hardware memory tagging support instead of compiler instrumentation and
> +shadow memory.
> +
> +Hardware tag-based KASAN is based on both arm64 Memory Tagging Extension (MTE)
> +introduced in ARMv8.5 Instruction Set Architecture, and Top Byte Ignore (TBI).

Is there anything inherently tying tag-based KASAN to arm64? I guess if
some other architecture supports MTE, they just have to touch arch/,
right?

You could reword to say that "Hardware tag-based KASAN is currently only
supported on the ARM64 architecture.

On the ARM64 architecture, tag-based KASAN is based on both ..."

Thanks,
-- Marco